One example of a self-piercing rivet setting machine is described in Japanese Patent Laid-Open No. 08-505087. FIG. 1 of the publication shows one example of a self-piercing rivet. The self-piercing rivet comprises a flange-shaped head and a leg extending downward from the head. When the self-piercing rivet is driven into workpieces, such as two body panels, by using a punch and a die, the edge of the leg is outwardly expanded and deformed while piercing (boring) the panels by the leg. In this manner, the panels are connected with each other between the outwardly expanded and deformed legs and the head. The self-piercing rivet is suitable for assembling an aluminum body to which welding is not applicable. As aluminum bodies are increasingly employed in automobile bodies to drive forward weight reduction, the demand for the self-piercing rivet would increase in the future. For example, for two body panels as the workpieces, the self-piercing rivet is arranged to allow the leg to penetrate one panel on the side of the flange, but not to allow the edge of the leg to penetrate the other panel on the edge side of the leg. Thus, in addition to the prevention of insufficient driving force, it is required to prevent excessive driving force causing the penetration of the leg edge through the other panel.
A currently used self-piercing rivet setting machine is provided with an apparatus for detecting a rivet driven stroke and a rivet driving load of a self-piercing rivet to determine setting defects corresponding to either insufficient or excessive driving force. The setting-defect detecting apparatus comprises a monitor as display means for displaying and plotting a normal upper limit curve defining the upper limit of a normal setting range and a normal lower limit curve defining the lower limit of the normal setting range on a graph having X-Y coordinates representing the rivet driven stroke and the rivet driving load, means for measuring the rivet driven stroke and rivet driving load of a self-piercing rivet during a rivet driving operation to plot them on the graph, more specifically to perform processing of input data from a pressure sensor, scale or the like to plot them on the graph, and detecting means for detecting whether the plotted value would go across either one of the normal upper and lower limit curves and deviate from the values between both the curves to detect the setting defects of the self-piercing rivet. For example, if the plotted value decreases lower than the normal lower limit curve, it will be determined as insufficient driving force. If the plotted value increases higher than the normal upper limit curve, it will be determined as excessive driving force.
FIG. 1 shows a graph on the monitor. The monitor displays and plots the graph having X-axis representing the rivet driven stroke of a self-piercing rivet (mm) and Y-axis representing a rivet driving load of the self-piercing rivet for the rivet driven stroke (kN (kilo Newton)). On this graph, reference curves A and B are plotted. The upper curve A is a characteristic curve, defining the upper limit of a normal setting range, i.e. a normal upper limit curve, and the lower curve B is a characteristic curve defining the lower limit of the normal setting range, i.e. a normal lower limit curve. If input data of the rivet driven stroke-to-the rivet driving load of a self-piercing rivet under a rivet driving operation are plotted between the normal upper limit curve A and the normal lower limit curve B, its rivet driving operation is determined as normal. If the plotted value goes across either the normal upper limit curve A or the normal lower limit curve B and deviates from the values between both the curves, it will be detected that the self-piercing rivet has been set in a defect state, and thus determined as a setting defect. The normal upper and lower limit curves A, B serving as reference curves are acquired on an experimental basis through self-piercing rivet driving tests. For acquiring the characteristic curves, their values can be determined on an experimental basis, and any suitable values representing a normal setting state may be used. The detected setting defect is indicated to an operator as an alert, and the setting machine stops its operation.
As above, in the conventional setting-defect detecting method, when the plotted value of the self-piercing rivet under the rivet driving operation goes across either the normal upper limit curve A or the normal lower limit curve B and deviates from the values between these curves, it is detected that a setting defect occurs. Then, this setting defect is indicated to an operator, and the setting machine stops its operation. However, it has been found that not all of actual setting defects could be detected by such a setting-defect detection. FIG. 1 shows a curve C of plotted values of a self-piercing rivet under a rivet driving operation. This curve increases substantially linearly between the normal upper limit curve A and the normal lower limit curve B. Under the conventional detecting method, this plotted value is determined as a normal setting state, but not as a setting defect. However, in this case, the self-piercing rivet was actually overturned in the punch of the self-piercing riveting machine, and the rivet driving operation could not be normally carried out. As seen from this fact, according to the conventional setting-defect detecting method, the setting defect caused by either excessive or insufficient driving force can be detected however, any other setting defect different from such setting defects cannot be detected. If the width between the normal upper limit curve A and the normal lower limit curve B is reduced to detect this different type setting defect, many acceptable self-piercing rivets will be detected as the setting defects, and this can be obstacle in the normal setting operation. This proves that setting defects of a self-piercing rivet cannot be sufficiently detected by the conventional setting-defect detecting method.
It is therefore an object of the present invention to provide a method and apparatus for detecting a setting defect of a self-piercing rivet, capable of detecting not only conventionally detectable setting defects of a self-piercing rivet, but also an additional setting defect different from the conventionally detectable setting defects.